The direction finding of electromagnetic transmission signals has been practiced for many years and has been achieved using a wide variety of antenna configurations and signal processing techniques. One such antenna configuration is a linear array of receiving antennas. Each receiving antenna has its own receiver circuit, and they are configured to simultaneously receive the transmission signal. At long distances, the radiating source appears as a point source, and the transmission signal from the point source appears as a planar wave-front to the linear array of a relatively small length.
The simultaneously received signals from each receiving antenna are processed to determine the phase difference of the received signals between a pair of the receiving antennas (baseline). The measured angle of arrival of the transmission signal is a function of the distances between the receiving antennas in the baseline, the frequency of the transmission signal, and the measured phase difference. Thus, if the distance between the receiving antennas of the baseline is known, and the frequency and the phase differences are obtained through measurements, then the angle of arrival of the transmission signal relative to the array can be determined. If the array position in inertial space is known, then the angular position of the emitter can be determined in any coordinate system.
A system of this type is called an interferometer direction finder. In general, interferometer direction finders are capable of high precision angle of arrival measurements, but the angle measurement has multiple ambiguities caused by the periodic nature of the phase difference.
Conventionally, multiple interferometer baselines must be used to resolve the ambiguities, known as multiple baseline interferometry. In multiple baseline interferometry, different observations of the angle of arrival by several baselines are correlated to remove the ambiguities of the angle of arrival. Disadvantages of this approach are that it requires the space to install multiple antennas, preferably in a coplanar geometry, and it requires extra antenna elements and receiver electronics to process the multiple baseline signals. Moreover, to obtain precision angle measurements for VHF (very high) and UHF (ultra high) frequencies (50 MHz to 200 MHz), a long interferometer baseline is needed. The greater the end-to-end antenna spacing, the more ambiguities are introduced. The greater the number of ambiguities, the greater the number of intermediate antennas which must be added. This represents a large hardware cost and is undesirable, especially, if the antenna array is to be installed in a limited space.
The direction finding problem is also difficult in practical application because the frequency of the incoming signal typically is not known before its reception at the direction finding system. In fact, the direction finding system may be required to operate over a very broad frequency band and an incoming signal may lie anywhere within the band. An approach in such a case is to implement the direction finding system as a channelized device and employ digital channelizers to divide the broad frequency band at each antenna into a number of plural frequency channels (each narrower than the broad frequency band) to be processed individually. The main problem with this approach is that a relatively large number of digital channelizers are required, i.e., one for each antenna.
The direction finding system can be combined with a parameter encoder system which measures various parameters including frequency, pulse width, etc. of the transmission signal. Alternatively, the signals received at the receiving antennas can be converted through a Fourier transformation, followed by a phase-sequence analysis to determine the transmission signal frequency. Again, additional external devices or processing circuits are necessary.
Moreover, if the direction finding antennas are broad-band, the angular coverage of the arrays is typically 90° (spatially) and four such arrays are needed to cover fully 360°.
Thus, there is a need for direction finding method and system capable of determining the angle of arrival with high accuracy using as few antennas as possible, and requiring no dedicated circuit for the signal frequency determination.